1. Field of the Invention
The present invention generally relates to a reference current generating apparatus for generating an electric reference current (which may hereinafter be referred to as a reference current), in order to generate an electric reference voltage (which may hereinafter be referred to as a reference voltage) in a semiconductor integrated circuit. More particularly, the present invention relates to a reference current generating apparatus which is capable of adjusting non-uniformity of a reference current which might occur due to, for example, an error in the accuracy of resistance ratio and the like which may occur during manufacture of the apparatus.
2. Description of the Related Art
A typical example of a conventional electric circuit for generating a reference current in order to generate a reference voltage is disclosed in Japanese Laid-open Patent Application Publication No. 2000-75947. A band gap reference circuit (BGR circuit) is shown in FIG. 3 of Japanese Laid-open Patent Application Publication No. 2000-75947, and includes an operational amplifier used to generate a reference current. The reference current is a combination of a constant current proportional to a thermal voltage, and a current proportional to a diode voltage. An operation bias current of the operational amplifier is generated using the reference current.
In addition, a band gap reference circuit is shown in FIG. 5 of Japanese Laid-open Patent Application Publication No. 2000-75947. The band gap reference circuit shown in FIG. 5 includes a current source transistor that generates reference current (1/R1*(Vbe+R1/R3*kT/q*LN(n)), which is proportional to 1/R1 of a band gap voltage (Vbe+R1/R3*kT/q*LN(n)) and which has no dependency on temperature. The band gap reference circuit generates a constant reference voltage (=R4/R1*[Vbe+R1/R3*kT/q*LN(n)] at a Vref terminal by flowing the reference current through a load resistor R4 (884 kΩ) connected to the current source transistor. The constant reference voltage is R4/R1 times as high as the band gap voltage, and has no dependency on temperature. It is to be understood that Vbe is a terminal voltage of a diode, R1 is a resistor of 2063 kΩ, R3 is a resistor of 393 kΩ, k is Boltzmann's constant, T is absolute temperature, q is units of electric charge, and n is diode capacitance ratio.
Nevertheless, the band gap reference circuit disclosed in Japanese Laid-open Patent Application Publication No. 2000-75947 has no means for adjusting error in specific accuracy which may occur due to mismatch of resistor R4 and resistor R1 in view of manufacture thereof, and an error in specific accuracy which may occur due to mismatch of resistor R3 and resistor R1 in view of manufacture thereof, due to mask misalignment, dispersion of impurity concentration and the like. This makes it difficult for individual elements to generate a constant reference voltage (=R4/R1*[Vbe+R1/R3*kT/q*LN(n)]) having no dependency on temperature.
In the meantime, FIG. 1 of U.S. Pat. No. 6,501,256 shows a means for adjusting a mismatch error in specific accuracy of resistors in view of manufacture thereof, in a band gap reference circuit for generating a reference current, by summing a constant current proportional to a thermal voltage and a current proportional to a diode voltage.
However, in order to adjust a mismatch error in specific accuracy of resistor R2 and resistor R1 due to manufacture thereof (resistance of an output resistor 170/R1*[Vbe+R1/R2*kT/q*LN(n)]), the resistance of resistor R2 in FIG. 1 of U.S. Pat. No. 6,501,256 is varied by selectively switching on or off MOS switches 312 through 328 that are connected in series to parallel unit resistors of the resistor R2, as shown in FIG. 3. It should also be understood that here Vbe is a terminal voltage of diode D2, R1 is resistor 122, R2 is resistor 124, k is Boltzmann's constant, T is absolute temperature, q is units of electric charge, and n is diode capacitance ratio. The on-resistance of the MOS switches has a temperature dependency, which is different from the temperature dependency of resistors r, 2r, . . . , 16r shown in FIG. 3. This has an effect on the constant reference voltage (the resistance of the output resistor 170/R1*[Vbe+R1/R2*kT/q*LN(n)]). It is accordingly difficult to generate a constant reference voltage having no temperature dependency. If the on-resistances of the MOS switches are designed to be greatly smaller than the parallel unit resistances of the resistor R2 in order to avoid this difficulty, a problem arises in that a layout area of the MOS switches becomes very large.